AR15 v. M16 BCG - A shooter's comparison
I recently secured what is essentially a mil-spec M16 bolt carrier group for my AR - the only thing that isn't mill spec is the lack of MPI, a fancy way of looking for cracks. I'm not loosing sleep over it, I'll put it that way. No, this will not make the gun function differently, still semi-auto, blah blah blah.
Here's the most common AR15 style bolt carrier groups you'll find, without any various coatings that some makers put on them.
Click the link for the pictures:
The first one, the SP-1 on top, isn't found too often in new guns these days. Century did a lot of these on USGI M16 parts, and hacked them up badly. Anyway, the firing pin in this cocks the hammer directly - not necessarily a horrible thing, but there's better methods out there. Also notice that the rear portion that engages the recoil spring has very little weight.
The 2nd down is almost identical to the M16 bolt carrier group except for the bottom portion near the rear is smaller than the M16 group, but larger than the SP-1 group. The hammer is also cocked by the bolt carrier, not the firing pin.
The 3rd down is the M16 bolt carrier group.
You'll notice that the biggest difference is the amount of metal in the tail of the carrier. This is due to the full auto sear on the M16 being tripped by this extra meat on the bottom rear portion of the bolt carrier, allowing the hammer to be released when the rifle is in battery. We all know what lawyers do to products, and I can only assume that this reduction of material/weight is the result of lawyers. There is NO functional difference if you put an M16 BCG in an AR15.
I personally have been running a bit of a hybrid since November from Del-Ton. The portion from the firing pin forward on the carrier looks like the 2nd down in the picture, but the 'tail' that engages the buffer/recoil spring looks like the SP-1. It's TiN coated for ease of maintenance, and has been working just fine for over 5K rounds. It was included in the rifle kit.
Anyways, at the range last night I ran 20 rounds through my factory AR15 BCG. I then switched over to the M16 bolt carrier group, and proceeded to shoot another 20 rounds. No difference in felt recoil. I put the factory BCG back in and did 20 rounds rapid fire, and got the usual minimal amount of dot motion. I popped the M16 BCG in there and did 20 rounds rapid fire - and the dot didn't move. The extra weight was moving slower, reducing the speed of the action when firing, and the motion of the rifle if I had to take a guess. The typical slight horizontal/vertical motion was gone, with the rifle recoiling straight back into me, allow for much, much faster sustained rapid fire without stopping. Approximately 1-1.5 seconds was reduced in the amount of time it took to rapid fire 20 rounds, and the grouping was slightly smaller.
The easiest way to describe the difference will likely make sense to those who have participated in an Appleseed or similar rifle course. Think back to watching the front sight post under recoil when you were in your NPA to call your shots, and how it moved when you were sitting. Now think about the same thing under prone. Remember how prone the rifle went back and then the front sight post was right where you wanted/needed it?
That's the difference that I personally experienced using the same ammunition, grip, stance, rifle, etc. - just changing out the bolt carrier group for the heavier M16 group.
To those that say it's in my head, you may be right, you may be wrong. But that doesn't matter - if the shooter THINKS it made a difference for them, it did :)